Constant voltage reset circuit for forward converter

ABSTRACT

A constant voltage reset circuit used in a forward converter for holdup time requirement or wide input voltage range is disclosed. In one embodiment, the constant voltage reset circuit comprises (1) a reset capacitor, and (2) a parallel-coupled switch and a diode coupled in series with the reset capacitor which is configured to cooperate therewith to reset an energy from the transformer. The forward converter employing the reset circuit or the method achieves high efficiency with large turn ratio and without 50% limitation of maximum duty cycle.

FIELD OF THE INVENTION

[0001] This invention relates generally to power supply systems that performs DC-DC conversion operations, and more particularly, this invention relates to a constant voltage reset circuit for forward converter to operate the converter more efficiently at holdup time requirement applications or wide input voltage range operating applications.

BACKGROUND OF THE INVENTION

[0002] DC-DC converters are most commonly used in various power supplies. The input of the DC-DC Converter can be a fixed DC voltage with pre-regulator or a wide range DC voltage.

[0003]FIG. 1a shows the conventional configuration and FIG. 1b to FIG. 1d show key operation waveforms of dual switch forward DC-to-DC converter. When the two switches S1 and S2 turn on simultaneously, the primary winding of the transformer Tr is magnetized by the input voltage Vin. And when the two switches S1 and S2 turn off simultaneously, the transformer winding is demagnetized reversely by input voltage Vin through two clamping diodes Da1 and Da2. The reset voltage is equal to the input voltage. In order to keep the voltage-second balance of transformer winding, the duty cycle of forward converter is limited within 50%.

[0004] A drawback associated with foregoing DC-DC converter is that the converter operates in a small duty cycle with low efficiency under normal high input, since the converter is designed at minimum input voltage with a limited maximum duty cycle of 50%. The smaller duty cycle, the smaller turn ratio of transformer, and thus the larger RMS current of the primary side and the larger the conduction loss and the switching loss of the main switches. Meanwhile, the smaller turn ratio of transformer, the higher voltage rating of second side rectifier with higher conduction voltage drop and the bigger size of output filter.

SUMMARY OF THE INVENTION

[0005] It is an object of the present invention to extend the switch duty cycle to ensure high conversion efficiency for wide input operation range.

[0006] It is another object of the present invention to minimize bus capacitor for a given holdup time requirement.

[0007] It is a further another object of the present invention to lower the voltage rating and thus to lower the conduction loss of second side rectifier.

[0008] To achieve these objectives and other advantages of the present invention and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention provides a constant voltage reset circuit for a forward converter. The constant voltage reset circuit comprises (1) a reset capacitor, and (2) a parallel-coupled switch and a diode coupled in series with the reset capacitor, which is configured to cooperate therewith to reset energy from the transformer.

[0009] The present invention introduces, in one aspect, the concept of constant voltage reset circuit employable on a transformer isolated power supply of forward converter. The constant voltage reset circuit is capable of demagnetizing the transformer and is further capable of recovering the energy stored in the transformer with a constant voltage reset to extend the switch duty cycle higher than 50% at runnable minimum input to improve the overall performance of the converter.

[0010] In one embodiment of the present invention, the constant voltage reset circuit further comprises a logic control unit coupled to the switch which is configured to enable the reset voltage of the forward converter to be almost constant even when the input of the forward converter drops to a lower voltage.

[0011] In one embodiment of the present invention, the diode configuration is selected from a separated diode, a zener diode and a body diode of the switch as a MOSFET.

[0012] In one embodiment of the present invention, a resistor is further connected in parallel with the diode.

[0013] In one embodiment of the present invention, the logic control unit includes a zener diode and a transistor biased through the zener diode.

[0014] Other objects, features and advantages of the present invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] For a more complete understanding of the present invention, references are now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0016]FIGS. 1a, 1 b, 1 c and 1 d show the configuration and key operation waveforms of a conventional dual switch forward DC-to-DC converter;

[0017]FIGS. 2a, 2 b, 2 c and 2 d show circuit diagrams and key operation waveforms of a particular type of constant voltage reset dual switch forward DC-DC converter according to a first preferred embodiment of the present invention;

[0018]FIG. 3 is a circuit diagram of a particular type of constant voltage reset dual switch forward DC-DC converter according to a second preferred embodiment of the present invention;

[0019]FIG. 4 is a circuit diagram of a particular type of constant voltage reset dual switch forward DC-DC converter according to a third preferred embodiment of the present invention;

[0020]FIG. 5 is a circuit diagram of a particular type of constant voltage reset dual switch forward DC-DC converter according to a fourth preferred embodiment of the present invention;

[0021]FIG. 6 is a circuit diagram of a particular type of constant voltage reset dual switch forward DC-DC converter according to a fifth preferred embodiment of the present invention;

[0022]FIG. 7 is a circuit diagram of a particular type of constant voltage reset dual switch forward DC-DC converter according to a sixth preferred embodiment of the present invention; and

[0023]FIG. 8 is a circuit diagram of a particular type of constant voltage reset dual switch forward DC-DC converter according to a seventh preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0024] Referring to FIGS. 2a, 2 b, 2 c and 2 d, the configuration and key operation waveforms of a particular type of constant voltage reset dual switch forward DC-DC converter according to a first alternate embodiment of the present invention is illustrated. The invention differs from the prior art described in FIG. 1a in terms of the reset means in which a constant voltage is applied to reset the transformer during a variable input bus by inserting a constant voltage reset circuit to a conventional dual switch forward converter, one of the clamping diode of forward converter connecting to one port of the reset circuit. The constant voltage reset circuit comprises a reset capacitor Cr and a parallel-coupled switch Sa and diode Da3, which are coupled in series with the reset capacitor Cr, and a control logic unit is coupled to the switch Sa to control the voltage of the reset capacitor Cr.

[0025] In normal of high input voltage operating mode, the switch Sa is controlled to turn on by the control logic unit, the reset capacitor Cr is shorted to input DC bus, the reset voltage is equal to bus voltage, the operation of the converter is same as that of a conventional dual switch forward converter and all magnetizing energy is fully recycled. When input voltage fails, DC bus drops, the control logic unit activates a holdup operating mode for the converter by turning off the switch Sa, the reset capacitor is separated from DC bus, and the reset voltage at capacitor Cr will not drop but keep unchanged as normal high input bus voltage. Thus, higher reset voltage is maintained when input bus voltage drops. The runnable maximum switching duty cycle Dmax can be expressed as: $D_{\max} = \frac{Vcr}{{{Vcr} + {{Vin}\quad \min}}\quad}$

[0026] Where Vcr is voltage of reset capacitor Cr, and Vin min is runnable minimum bus voltage.

[0027] As an example, the bus voltage at normal operation is 72V, the runnable minmum bus voltage is 36V, and the reset voltage Vcr is 72V, the maximum limited duty Dmax can be extended to 66.7%. Therefore with the constant voltage reset, the turn ratio of the converter can be selected larger than that of a conventional dual switch forward converter. The larger turn ratio of transformer, the larger operating duty cycle, and thus the smaller RMS current of primary side and the smaller the conduction and the turn off loss of main switches. Meanwhile, the larger turn ratio of transformer, the lower voltage rating of second side rectifier with lower conduction voltage drop and the smaller size of output filter. The present invention promises a significant efficiency improvement in DC/DC converter.

[0028]FIG. 3 is another configuration of the present invention. As shown in FIG. 3, the constant voltage reset circuit comprises a reset capacitor Cr connected between DC bus and a parallel-coupled switch Sa and diode Da3, which connects another terminal to ground, and a control logic unit is coupled to the switch Sa to control the voltage of the reset capacitor Cr. This configuration is easy for design of gate driving of switch Sa.

[0029]FIG. 4 shows another configuration by replacing diode Da3 with a zener diode Za in the constant voltage reset circuit. The zener diode Za can limit the voltage difference between the reset voltage and the DC bus voltage during dropping of input bus.

[0030]FIG. 5 shows another configuration by connecting a resistor Ra in parallel with diode Da3 in the constant voltage reset circuit. The resistor Ra keeps the reset voltage in a certain range by discharging part of magnetizing energy to DC bus when input bus drops.

[0031]FIG. 6 shows another configuration of embodiment of the logic unit of the constant voltage reset circuit. This voltage reset circuit keeps a constant voltage reset in both normal operation and holdup operation by replacing switch Sa and logic unit with a biased transistor and a zener diode respectively.

[0032]FIG. 7 is a circuit diagram of another configuration of this invention as applied to a particular type of constant voltage reset single switch forward DC-DC converter. Compared with conventional single switch forward converter with third winding reset, this converter connects the third reset winding to reset port of a constant voltage reset circuit via a diode Da1. The reset circuit shown in FIG. 7 is the same as that of FIG. 2a, and can be replaced by those shown in FIG. 3, FIG. 4, FIG. 5 and FIG. 6.

[0033]FIG. 8 is a circuit diagram of this invention as applied to a particular type of multi-module DC/DC converter. Two or more converter modules share the same constant voltage reset circuit by organizing the reset diode to the same reset terminal. The constant voltage reset circuit shown in FIG. 8 is same as that of FIG. 2a, and can be replaced by any kind of reset circuit shown in FIG. 3, FIG. 4, FIG. 5 and FIG. 6.

[0034] Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is not to be interpreted as limiting. Various alternations and modifications will no doubt become apparent to those skilled in the art after reading the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alternations and modifications as fall within the true spirit and scope of the invention. 

What is claimed is:
 1. A constant voltage reset circuit for use in a forward converter having a transformer, comprising: a reset capacitor; and a parallel-coupled switch and a diode coupled in series with said reset capacitor, which is configured to cooperate therewith to reset an energy from said transformer.
 2. The reset circuit as recited in claim 1 wherein said reset circuit further comprises a logic control unit coupled to said switch which is configured to control a reset voltage of said voltage reset circuit to keep said reset capacitor with a sufficient reset voltage when an input power of said forward converter drops to a certain voltage.
 3. The reset circuit as recited in claim 1 wherein said diode is selected from a group consisting of: a single diode; a body diode of a metal-oxide-semiconductor field-effect transistor; and a zener diode.
 4. The reset circuit as recited in claim 3 wherein a resistor is further connected in parallel with said diode.
 5. The reset circuit as recited in claim 2 wherein said logic control unit comprises a zener diode and a transistor which is biased through said zener diode.
 6. A method of resetting a transformer of a forward converter comprising: charging a reset capacitor to a voltage level equal to a power source of said forward converter; and resetting said transformer of said forward converter by a parallel-coupled switch and a diode coupled in series with said reset capacitor, which is configured to cooperate therewith to reset an energy from said transformer.
 7. The method as recited in claim 6 wherein said method further comprises controlling a reset voltage of said voltage reset circuit by a logic control unit coupled to said switch to keep said reset capacitor with a sufficient reset voltage when input power of said forward converter drops to a certain voltage.
 8. The method as recited in claim 6 wherein said diode is selected from a group consisting of: a single diode; a body diode of a metal-oxide-semiconductor field-effect transistor; and a zener diode.
 9. The method as recited in claim 8 wherein a resistor is further connected in parallel with said diode.
 10. The method as recited in claim 7 wherein said logic control unit comprises a zener diode and a transistor which is biased through said zener diode.
 11. A forward converter for being coupled to a DC power source, comprising: at least one power switch that turns on alternately to transfer an energy from said DC power source to an DC output of said converter; a transformer having a primary winding coupled in series with said power switch; a voltage reset circuit coupled across said power switch and said primary winding and to one terminal of said primary winding, including: a reset capacitor, a parallel-coupled switch and a diode coupled in series with said reset capacitor, which is configured to cooperate therewith to reset an energy from said transformer; and a logic control unit coupled to said switch to keep said reset capacitor with a sufficient reset voltage when input power of said forward converter drops to a certain voltage.
 12. The forward converter as recited in claim 11 wherein said DC power source can be either an output of a battery, a power factor correction converter (PFC) or a DC/DC converter.
 13. The forward converter as recited in claim 11 wherein said reset circuit further comprises a logic control unit coupled to said switch which is configured to control a reset voltage of said voltage reset circuit to keep said reset capacitor with a sufficient reset voltage when an input power of said forward converter drops to a certain voltage.
 14. The forward converter as recited in claim 11 wherein said diode is selected from a group consisting of: a single diode; a body diode of a metal-oxide-semiconductor field-effect transistor; and a zener diode.
 15. The forward converter as recited in claim 14 wherein a resistor is further connected in parallel with said diode.
 16. The forward converter as recited in claim 13 wherein said logic control unit comprises a zener diode and a transistor which is biased through said zener diode. 